Early clues point to an unexpected hazard in the deadly midair engine rupture and emergency landing of Southwest Flight 1380: the engine cover.

The Southwest Airlines Co. accident that killed a passenger Tuesday sent fast-moving debris into the jet’s left wing, exterior of the plane’s body and inside the passenger cabin. Safety experts are asking whether U.S. regulators and engine makers have underestimated the role of the engine cover in the unlikely event engine parts break loose in flight.

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